One type of drill bit used in forming a borehole in the earth is a roller cone rock bit. A typical roller cone rock bit comprises a body with an upper end adapted for connection to a drill string. A plurality of arms, typically three, depend from the lower end portion of the body. Each arm includes a spindle protruding radially inward and downward with respect to a projected rotationally axis of the body. Each cutter cone also defines an internal cavity shaped to receive an associated spindle. A cutter cone may be mounted on each spindle and rotatably supported on bearings acting between the spindle and the inside of the cavity in the cutter cone. One or more nozzles often are located on the underside of the body and radially inward of the arms. These nozzles are generally positioned to direct drilling fluid passing downwardly from the drill string to the bottom of the borehole being formed. The drilling fluid washes away the material removed from the bottom of the borehole and cleanses the cutter cones carrying the cuttings radially outward then upward within the annulus defined between the bit body and the wall of the borehole.
Protection of the bearings that allow rotation of the cutter cone can lengthen the useful service life of a roller cone rock bit. Once drilling debris or external fluids are allowed to infiltrate between the bearing surfaces of the cutter cone and the spindle, failure of the roller rock bit will follow shortly. Various mechanisms are employed to keep debris and external fluids from entering between the bearing surfaces. A typical approach is to utilize an elastomer seal across the gap between the bearing surfaces of the rotating cutter cone and its support on the spindle. However, when this seal fails, it is not long before external fluids or drilling debris contaminate the bearing surfaces through the gap between the cutter cone and the spindle. Thus, it is important that the seal be fully protected against wear caused by debris in the borehole and protected from pressure exerted by external fluids, such as water present in the borehole.
In a sealed roller cone rock bit, the cavity in the cutter cone and the bearings are lubricated by packing the cavity and bearings with a lubricant such as grease. This lubricant is sealed from the external environment by the elastomer seal across the gap between the cutter cone and the spindle. It is important that the lubricant not be contaminated with external fluids or debris. This seal is especially important when the roller cone rock bit is used in drilling a deep well, such as a deep oil or gas well, where the borehole is filled with a column of water or other liquids exerting tremendous pressure on the seal. In such a high pressure downhole environment, the external fluids exert constant pressure on the outer side of the seal. If the seal fails, external fluids can mix with the lubricant and quickly cause failure of the roller cone rock bit. Thus, it is important to maintain the integrity of the seal.
A safeguard used in some roller cone rock bits is a second seal outward from the first elastomer seal. The addition of a second seal creates a seal gap between the second seal and the first seal. The second seal acts as an initial barrier to the external fluids and debris. Examples of a roller cone rock bit having a second outward seal are described in U.S. Pat. No. 4,981,182 and U.S. Pat. No. 5,027,911. Both of these patents show an outer seal utilizing a Belleville spring washer molded with a rubber or elastomer shroud. Such flat face seals are often used in roller cone rock bits.
At assembly of the roller cone rock bit and arm parts, the Belleville spring is compressed and thereby imparts a force upon the elastomer shroud which effects the sealing function. This flat seal is presently in widespread use in roller cone rock bits. Originally, such seals were the principal sealing methods used in roller cone rock bits. They have been replaced in recent years by ring-type shaft seals, such as O-rings and similar elastomer ring seals. Recently, however, the flat seal has been used as the outer protective member of a double seal group in conjunction with an inner seal. Use of a flat seal as an outer barrier is shown in the above patents.
In current usage, the elastomer shroud of the flat seal must stay intact for there to be an effective sealing function and for the flat seal to protect the inner seal. When the elastomer shroud is worn or abraded due to debris in the borehole, then the Belleville spring washer can no longer exclude borehole debris from reaching and acting upon the main inner seal because the Belleville spring by itself does not form a seal. The elastomer eventually is worn to the extent that a seal is no longer affected and debris is allowed to attack the integrity of the inner seal.